Triacylglycerols are storage vehicles of fatty acids (primary energy source) and phosphatidylcholines are integral components of membranes, bile and lipoproteins. Yet, these glycerolipids share a common pathway in which diacylglycerol is utilized in their final biosynthetic step. Therefore, understanding the regulation of the "diacylglycerol branchpoint" is fundamental to determining how the liver cell alters the quantity of triacylglycerol and phosphatidylcholine that is necessary to meet the cell's varied and simultaneous requirements of lipoprotein synthesis, membrane proliferation and biliary phospholipid formation. These studies will demonstrate that: 1) agent-dependent changes in phosphatidate phosphatase and phosphocholine cytidylyltransferase activity are key determinants of liver cells triacylglycerol and phosphatidylcholine biosynthesis, respectively; 2) these regulatory enzymes (cytidylyltransferase and phosphatase) are in a "dynamic equilibrium" with the cytosol (reservoir) and endoplasmic reticulum such that they are bound or released as needed by the endoplasmic reticulum to rapidly alter glycerolipid biosynthesis; 3) diacylglycerols are preferentially incorporated into phosphatidylcholine when CDP choline is present and triacylglycerol when CDP choline is not available. The first two objectives will be achieved by demonstrating that the oleate- and clofibrate-dependent, preferential increases in cultured hepatocyte triacylglycerol and phosphatidylcholine formation and content, respectively, are primarily a result of selective increases in the activity of microsomal phosphatidate phosphatase and phosphocholine cytidylyltransferase. The mechanisms by which oleate- and clofibrate-preferentially increase microsomal phosphatase and cytidylyltransferase activity, respectively, will be determined in studies using antibodies of these purified proteins. The final objectives will be accomplished by determining the influence of CDP choline on the incorporation of labeled diacylglycerols into triacylglycerol and phosphatidylcholine in microsomes and saponin permeabilized hepatocytes.